libstdc++: Fix versioned namespace build

libstdc++: Fix versioned namespace build

The recent changes to define various std::exception_ptr functions inline
included a change so that the definitions of those functions would be
omitted for the ABI unstable gnu-versioned-namespace configuration. That
change was incorrect, because the existing functions that are gated by
the _GLIBCXX_EH_PTR_COMPAT macro are always needed even for the
versioned namespace.

This change introduces a new macro to control whether operator== is
defined as deleted or not, distinct from the existing macro. The new
macro is not defined for versioned namespace builds, but the old macro
still is.

libstdc++-v3/ChangeLog:

	* libsupc++/eh_ptr.cc (_GLIBCXX_EH_PTR_RELOPS_COMPAT): Define
	new macro.
	* libsupc++/exception_ptr.h (_GLIBCXX_EH_PTR_USED): Check new
	macro instead of _GLIBCXX_EH_PTR_COMPAT.
	(operator==): Likewise.